1. Field of the Invention
This invention relates to an actuator apparatus for driving a load or the like by utilizing the shape restoring function of shape memory alloy (SMA), and for example, to a memory shape actuator for use in a mechanism for bending the insertion end portion of an endoscope.
2. Description of the Related Art
In a general bending method used in the endoscope, a bending wire is inserted through the inserted portion with the tip end thereof fixed on the insertion end portion and the rear end connected to the knob of a manually operating section, and the knob is manually operated to pull or push the wire so as to bend the insertion end portion.
The above bending method has various problems that the construction is complex, the diameter of the insertion section is made large and a relatively large force is required to operate the knob. In order to solve these problems, a new method has been developed in which SMA is set inside the bending end portion of the endoscope and SMA is heated to show the shape restoring function by passing current therethrough. Such a method is disclosed in, for example, Japanese Patent Disclosure 59-48710 and Japanese patent application 61-276089 filed by the same applicant as that of this invention.
It is necessary to detect the bending degree of SMA in order to control the amount of current to be supplied to SMA. Since, in general, the resistance of SMA varies with the displacement thereof, the bending degree of SMA can be detected by detecting the resistance thereof.
In the prior art, a resistance detector is connected to a lead wire for heating SMA by passing current via SMA in order to detect the resistance of SMA, for example. Such a technology is disclosed in Japanese Patent Disclosure 62-26041. However, in the case where current is passed via SMA to heat the same, the lead wire is also heated and the resistance thereof is also changed. As a result, the detected resistance is influenced by variation in the resistance of the lead wire which is heated by passing current therethrough, and therefore it is impossible to precisely measure the resistance of SMA.
In many cases, the resistance detector is provided separately from the main body of the endoscope, e.g., in a light source unit, and the length of the insertion section and light guide cable of the endoscope may differ depending on the type of the endoscope. Therefore, the length of the lead wire differs depending on the type of the endoscope and the influence by the resistance of the lead wire on the measured resistance of SMA may vary depending not only on the temperature but also on the type of the endoscope. Further, SMA itself may have different resistances. Therefore, when a different endoscope is used, the detected resistance may vary, and it will be impossible to selectively connect a common light source unit to a plurality of endoscopes.
Further, since variation in the resistance of SMA due to the phase change of SMA is extremely small, it is difficult to precisely detect the resistance.
Therefore, it has been considered to amplify the resistance variation caused by the phase change. In general, in order to increase the resistance of SMA, the cross-sectional area thereof is reduced. However, in this case, the bending force by SMA will be weak. In a method disclosed in Japanese Patent Disclosure 60-175777, a plurality of coils of SMA are arranged in parallel in order to attain a sufficiently large bending force using SMA with a reduced cross-sectional area. However, in this case, it is necessary to provide a large space for the plurality of SMA coils, increasing the diameter of the insertion end portion.